1. Field of Invention
The invention relates to fiber optic sensors, and more particularly to fiber optic sensors for detecting the fiber's position and orientation, and still more particularly to fixtures used to constrain a fiber optic sensor with reference to a link in a kinematic chain.
2. Art.
In shape sensing systems that use interferometric techniques to interrogate an optical glass fiber with refractive index change (Bragg) gratings, the physical resources of the interrogating hardware limit the number of data points that can be used to describe the fiber's shape at any particular time. Specifically, the product of the capacity of the interrogator's electronic data processor and the interrogating laser's frequency sweep range is approximately proportional to the product of the length of the fiber being sensed and the peak strain that occurs within the sensed length. Bending is typically the primary source of strain in an optical fiber used as a shape sensor. The peak strain effectively occurs at the point of minimum bend radius in the sensed portion of the fiber.
In order to use a fiber optic shape sensor in a kinematic chain, such as a robot manipulator arm, it is often useful to constrain one or more portions of the sensed length in known positions and/or orientations relative to the individual links in the chain. Known ways of holding an optical fiber in place include the use of collets and various other mechanical clamping mechanisms, as well as gluing, etc. When applied to a moveable joint, however, many of these methods of holding a fiber result in a point load being applied to a location along the fiber, which results in exceedingly large peak strains at that location. What is required is a way to effectively eliminate these large peak strains.